Method and device for executing control and monitoring measurements in optical transmission paths

ABSTRACT

A method and a system for carrying out control and monitoring measurements on optical transmission paths which can be used during regular operation and without disturbing the same to measure and control parameters of the transmission path as well as to monitor the transmission path with respect to tapping attempts. A control signal having wavelength that is different from the transmission signal into the transmission path, re-reflected after propagating through the transmission path and analyzed. To reflect the control signal, a wavelength-selective reflecting set-up, such as a dichroic reflector is used, which transmits the transmission signal substantially as an undisturbed signal. The reflected control signal is detected by a detecting device and analyzed with respect to polarization, intensity, signal shape, or other properties, making it possible to infer the properties of the transmission path and/or detect unwanted tampering with the transmission system.

FIELD OF THE INVENTION

The present invention relates to a method and a system for carrying outcontrol measurements on optical transmission paths. The presentinvention further relates to a method and a system for carrying outmonitoring measurements on optical transmission paths.

BACKGROUND OF THE INVENTION

In view of the high business use of optical transmission, the monitoringof the security of the transmission paths can be a critical ability. Toprovide anti-tapping and data-protection security, methods are knownwhich assume substantial experimental outlay, for example, monitoringmonomode fibers using cryptologic such as quantum cryptologic methods,or utilizing Raman or Brillouin backscattering. These very expensivemethods may document the considerable interest that exists in achievingunaffected, trouble-free communication transmission.

In addition, regular control and checking of optical transmission pathsare needed to maintain proper telecommunications operation via opticaltransmission paths. Special cable fibers or glass fibers can requirecontinuous monitoring. Thus, the operational path must be able to becontrolled in parallel with the normal telecommunications operation,without disturbing this operation.

French Patent No. 2 739 992, describes a monitoring system based on theanalysis of reflected signals for an optical telecommunications network.This monitoring system makes use of Bragg gratings.

European Patent No. 0 432 734, describes a device and a method fordetecting defective locations or damage on an optical transmissionsystem.

Those systems appear to work with an additionally launched signal. Thelaunched signal can be selectively reflected after propagating throughpart of or all of the entire transmission path and whose wavelength isdifferent from the wavelength of the useful information to betransmitted.

However, those known systems are not suited for utilizing the chromaticdispersion that the signals propagating through the fiber are subject toin order to control the optical transmission path. Further, those knownsystems are not suited for utilizing the polarization state that thesignals exhibit after propagating through the fiber in order to monitorthe optical transmission path.

SUMMARY OF THE INVENTION

The present invention provides a method and a system for carrying outcontrol measurements during normal telecommunications operation, withoutadversely affecting the same, and the chromatic dispersion, which thesignals propagating through the fiber are subjected to, is utilized tocontrol the optical transmission path.

The present invention further provides a method and a system forcarrying out monitoring measurements during normal telecommunicationsoperation, without adversely affecting the same, and the polarizationstate exhibited by the signals after propagating through the fiber andtheir propagation time are analyzed to monitor the optical transmissionpath.

The present invention further provides minimizing the technical outlayto a minimum. The method and the system, respectively, should make itpossible to acquire important operating parameters for the transmissionpath and, additionally, to reliably detect any tampering with thetransmission path, especially tapping of or eavesdropping on the datacommunication traffic.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows the principal design layout of a system for making controland monitoring measurements in an embodiment of the present invention.

FIG. 2 shows schematically, a tested, controlled transmission path,which is coupled to a further transmission path in another embodiment ofthe present invention.

FIG. 3 shows OTDR measurements on a transmission path of anotherembodiment of the present invention.

FIGS. 4a shows fluctuations in the Stokes parameters of aninterference-free fiber line as a function of time in another embodimentof the present invention.

FIG. 4b shows fluctuations in the Stokes parameters of an interferedfiber line as a function of time in another embodiment of the presentinvention.

FIG. 4c shows a depiction of the polarization state of the reflectionsignal on the Poincaré sphere in the interference-free state in anotherembodiment of the present invention.

FIG. 4d shows a depiction of the polarization state of the reflectionsignal on the Poincaré sphere in the interfered case in anotherembodiment of the present invention.

FIG. 5 shows the polarization mode dispersion of a fiber as a functionof the wavelength in the interference-free and interfered states,respectively, in another embodiment of the present invention.

DETAILED DESCRIPTION

Transmission paths are, for example, individual optical fibers or fiberbundles combined to form a data cable, or also already networkedfibers/cables, or those which are interrupted by one or a plurality ofadditional elements, such as line repeaters. The transmission path hasan input and an output, which, here, define the transmitting andreceiving side, respectively. On the transmit side, data are encoded inthe form of optical signals (transmission signals), for which purpose atransmitter having an operational wavelength λ₁ is used, and arelaunched into the transmission path. It is not necessary for thetransmitter of the transmission signal to be located in the immediatevicinity of the transmit-side input of the transmission path; othertransmission elements can be connected therebetween. On the receiveside, the data are decoupled or extracted from the transmission path,analyzed, or routed to other transmission paths.

In this context, the operational wavelength is not necessarily exactly afixed wavelength due to the time modulation, transmission signals have acertain bandwidth, operational wavelength being understood to be thefundamental wavelength of the transmission signal. In addition, thetransmission path can be used in wavelength division multiplex operation(“WD. operation”). In the process, a plurality of signals havingdifferent fundamental wavelengths, which each encode different, mutuallyindependent pieces of information, are superimposed on the transmit sideto form a transmission signal, are transmitted, and separated again onthe receive side. Therefore, according to the present invention, theoperational wavelength λ₁is the wavelength range in which data aretransmitted in the case of the transmission path to be monitored.

In an embodiment of the present invention, an optical control signal isgenerated having a control wavelength λ₂, which is different fromoperational wavelength λ₁. λ₂ is likewise to be viewed as thefundamental wavelength of the control signal, which itself can have acertain bandwidth. In addition, at the same time, a plurality of signalshaving different wavelengths can be generated, which, along the lines ofthe present invention, can be coupled together to form a control signalin the manner of the WD. operation.

The system in accordance with the present invention has a transmitterfor producing the control signal, which, in some instances, can be usedapart from the transmitter for producing the transmission signal. On thetransmit side, the control signal can be launched into the transmissionpath. In this context, the coupling mechanism can be selected so as toenable a parallel launching of the transmission signal. For this, thesystem according to an embodiment of the present invention may include acoupler, which is configured on the transmit side in the area of thetransmission path and which is able to launch the signals from bothtransmitters into the transmission path. The coupler can be a multiplexcoupler, for example, as also used for WD. operation (for example, aY-coupler). Due to the different wavelengths, the transmission signaland, thus, the regular coupling of data may not be disturbed.

The control signal can be transmitted over the transmission path,reflected on the receive side, and retransmitted over the transmissionpath, back to the transmit side. For this, the system in accordance withthe present invention has a reflecting set-up, which is configured onthe receive side of the transmission path and is able to reflect thecontrol wavelength, the re-reflected control signal (reflection signal)being launched on the receive side again into the transmission path.

The reflecting set-up can includes, for example, a dichroic reflector ora dichroic layered reflector system, which transmits radiation in therange of the operational wavelength and reflects radiation in the rangeof the control wavelength, and is arranged at the output of thetransmission path. A reflector of this kind, adapted to the particularwavelengths, can allow the transmission signal to pass through with onlylittle attenuation, so that the regular data transmission is notdisturbed. The same applies to a reflecting set-up, which includes amultiplex coupler, and can be used in this embodiment of the presentinvention as a demultiplexer and as having at least twowavelength-dependent outputs and one reflector that is highly reflectingfor the radiation in the range of the control wavelength. The output ofthe multiplex coupler for radiation in the range of the operationalwavelength can be coupled to a receiver for transmission signals or toanother transmission path; the highly reflecting reflector is configuredat the output for radiation in the range of the control wavelength.

In the next method step, the re-reflected control signal (reflectionsignal) on the transmit side can be decoupled from the transmission pathand fed to a detecting device. The launching of the control andtransmission signals may be essentially unaffected by the decoupling ofthe reflection signal. In some instances, decoupled light of theoperational wavelength λ₁, is suppressed.

The reflection signal can be decoupled, for example, by the (multiplex)coupler used for launching, which acts as a demultiplexer for thereflected beam. In another embodiment of the present invention, thesystem for decoupling the reflection signal can include a circulatorwhich is arranged in the area of the transmit side end of thetransmission path. The circulator can be used for the transmit sidelaunching of the already multiplexed transmission and control signalinto the transmission path and for the transmit-side decoupling of thereflection signal out of the transmission path. The reflection signalcan be fed to the detecting device.

Finally, according to an method embodiment of the present invention, thedetecting device can be configured to emit an output signal which isindicative of the intensity and/or of the polarization state, in someinstances as a function of time, and/or the signal propagation time ofthe reflection signal, from which one can derive transmission propertiesof the transmission path and/or changes in the transmission properties,in particular those caused by tampering with the transmission path;through analysis of the reflection signal, one determining the chromaticdispersion of the fiber, in particular the zero dispersion wavelengthλ₀, the rise in the dispersion curve at λ₀, and/or the dispersion valuesin the second or third optical window.

The second optical window is understood here to be the spectral range of1,280 nm to 1,350 nm; the third optical window is understood to be thatof 1,480 nm to 1,700 nm.

For this, the detecting device may include a detector which is able todetect radiation of the control wavelength and which emits anintensity-dependent electrical output signal. The detecting device mayadditionally include at least one polarization-sensitive element, suchas a polarizer, a polarization beam splitter, a polarizing fibercoupler, in particular, however, a polarimeter for detecting at leastone component of the Stokes vector, preferably of the entirepolarization state of the reflection signal. In addition, the detectingdevice can include a data processing system for further analysis of thedetector signals.

Finally, according to another embodiment of the present invention, thedetecting device can be configured to emit an output signal which isindicative of the intensity and the polarization state, in someinstances as a function of time, and the signal propagation time of thereflection signal, from which one can derive transmission propertiesand/or changes in the transmission properties of the transmission path,in particular those caused by tampering with the transmission path.

For this, the detecting device can include a detector, which is able todetect radiation of the control wavelength and which emits anintensity-dependent electrical output signal. The detecting device canalso include at least one polarization-sensitive element, such as apolarizer, a polarization beam splitter, a polarizing fiber coupler, ora polarimeter for detecting at least one component of the Stokes vector,of the entire polarization state of the reflection signal. In addition,the detecting device can include a data processing system for furtheranalysis of the detector signals.

The methods according to the present invention may be used forcontrolling and monitoring the security of installed telecommunicationscables or individual fibers of the same, continually or for certain timeperiods, without entailing substantial outlay. In the embodimentsaccording to the present invention, the control or monitoringmeasurements can be made on the transmit side of the transmissionsystem, where generally the transmission system is also installed. Thespace requirements of the system according to the present invention canbe quite modest and having low technical complexity.

The embodiments according to the present invention can also provide thatthe control and monitoring measurements in no way impede the normal datatransfer; that is, the normal data transfer can continue in parallel.Further, the optical components of the present invention areinexpensive, so the economic outlay remains modest as compared to otherknown methods.

FIG. 1 depicts the principle of a design layout in accordance with thepresent invention to be used for taking control and monitoringmeasurements on an optical transmission path 101 or 111. FIG. 1brepresents an alternative way for implementing the present invention onthe receiving side in accordance with FIG. 1a. The left (transmit side)part of the system in accordance with the present invention from FIG. 1bcorresponds to that from FIG. 1a.

Besides the optical transmission signal having operational wavelengthλ₁, which is generated by a transmitter 102, a control signal havingcontrol wavelength λ₂ is generated by a second transmitter 103 and islaunched into a coupler 114. Coupler 114 is a broadband Y-coupler, e.g.,a multiplex coupler. On the output side, coupler 114 is linked to acirculator 104 via its port P1. Port P2 of circulator 104 is coupled,e.g., intermateably connected or spliced, to the input of transmissionpath 101 to be tested. Linked to third port P3 of circulator 104 isdetecting device 105, which, here, is made up of a monitoring measuringinstrument 106 and a control measuring instrument 107 for carrying outcontrol and monitoring measurements.

The circulator has the property of allowing light coupled into port P1to pass substantially with low loss non-dissipatively to port P2, but tosignificantly attenuate the transmission to port P3. On the other hand,light coupled into port P2 is directed to port P3, the transmission fromP2 to P1 being significantly attenuated. Consequently, circulator 104functions as an optical valve, which directs radiation into variousoutput channels, depending on the direction of propagation.

Mounted, respectively, at the output of transmission path 101 and 111 isa reflecting set-up 108 and 112 used for reflecting the control signalhaving control wavelength λ₂. In FIG. 1a, reflecting set-up 108 includesa short glass fiber segment 110, on whose back end a dichroic reflector109 is mounted. The front end of glass fiber segment 110 is provided,for example, with an optical connector, which is used to intermateablyconnect it to the receiving-side output of the transmission path.Dichroic reflector 109 reflects in the range of control wavelength λ₂,preferably with a reflection coefficient of more than 95%, ideally ofmore than 99%, and transmits in the range of operational wavelength λ₁,preferably with a transmittance of at least 95%.

Dichroic reflector 109 can be integrated in a special connector, e.g.,by adhesively mounting a dichroic layered reflector system on the endface of the connector. Dichroic layered reflector system can be designedto allow operational wavelength λ₁, to pass with a high transmittancethrough the reflector. The transmission signal is then fed to thefollowing cable section; see, e.g., FIG. 2, or received and analyzed bya receiver. On the other hand, control wavelength λ₂ is reflected with ahigh efficiency. With the aid of the only slightly attenuatedoperational wavelength, the operating traffic is sustained; on the otherhand, the reflected control wavelength is used on the transmit side tocontinuously monitor transmission path 101, as well as for controlmeasurements.

An alternative implementation of a reflecting set-up 112 is illustratedin FIG. 1b. In this case, reflecting set-up 112 includes a coupler 113,which functions as a demultiplexer. The transmission signal havingoperational wavelength λ₁, is fed via one arm of coupler 113 to the nextcable section or to a receiver. The control signal, on the other hand,is carried in the second arm of the coupler, at whose output is mounteda reflector 115 which is highly reflective—at least in the range of thecontrol wavelength. Reflector 115 is preferably designed as a highlyreflective fiber connector, which is coupled into the correspondingoutput of demultiplexer 113. The control signal is reflected off of thisreflector 115 in such a way that it is again launched, on thereceive-side, into transmission path 111, arrives at port P2 ofcirculator 104 and, finally, is transmitted to detecting device 105.

The reflected radiant power is decoupled at port P3 of circulator 104and supplied as a reflection signal to detecting device 105, i.e., toone of measuring instruments 106, 107. On the basis of the reflectionsignal, all types of control and monitoring measurements can be carriedout without the actual telecommunications operation being interferedwith or affected. Coupler 114 can be broadband and suited forwavelengths in the range of 1,300 to 1,650 nm. The junction losses arepreferably less than 0.2 dB. The present invention may also employ acirculator, including a circulator which is broadband. A circulator canbe used for each of the two optical windows. The junction losses in thefirst case are preferably less than 0.8 dB, in the second case less than0.6 dB. The insulation values for a transmission from port P2 to P1, andport P3 to P2 are preferably greater than 35 dB in the first case,preferably greater than 45 dB in the second case. The directivitypreferably amounts to at least 60dB, in accordance with a very highinsulation on the direct path from port P1 to P3.

Depending on the operating system, operational wavelength λ₁, is withinthe second or third optical window, thus around 1,300 or 1,550 nm. Thereare no fundamental restrictions for control wavelength λ₂. The controlwavelength is preferably greater than the operational wavelength, i.e.,greater than 1,600 nm. When working with conventional, standard opticalfibers having a critical wavelength of about 1,300 nm, in the case ofsuch a large wavelength, a considerable portion of the control signal ispropagated in the fiber cladding. This makes the fiber very sensitive tobends in the fiber, i.e., in the cable, thereby increasing thesensitivity of the measuring process. To avoid high fiber attenuation,too large of a control wavelength should not be selected. It should liepreferably within the third optical window.

Transmitters 102 and 103 for the transmission signal, i.e., the controlsignal, are radiation sources which produce optical signals having ahigh beam quality. Infrared lasers can be used, such as dye lasers orlaser diodes. The control signal can be generated in pulse or continuouswave operation. In some instances, additional information can also bemodulated on the control signal, for example, for bit error ratemeasurements. To carry out wavelength-dependent measurements, thecontrol signal can be tuned, also with respect to the controlwavelength, e.g., to measure the spectral polarization mode dispersion(see FIG. 4). For this, thermally tunable DFB semiconductors or tunableexternal cavity lasers are used, for example.

In the connection between port P2 of circulator 104 and transmissionpath 101, as well as in the case of all other optical couplings withinthe propagation path of the transmission signal, the aim is to avoidreflections. For this, the optical connections can be designed asspliced joints. If a plug connection is needed, then immersion oilshould be used inside the connector. The reflection at the couplingpoints preferably amounts to less than 1%.

FIG. 2 schematically depicts how an examined, controlled transmissionpath 201, which forms a first cable section, is linked via a dichroicreflector 203 to another transmission path 204, which forms a secondcable section. In this context, as in FIG. 1a, dichroic reflector 203 isthe main component of reflecting set-up 202, which reflects controlwavelength λ₂, but transmits the actual transmission signal havingwavelength λ₁, as a substantially unaffected signal, to the second cablesection. In place of second transmission path 204, any other elementused on the receiver side in optical transmission systems, can bemounted behind dichroic reflector 203.

The optical set-up in accordance with FIGs. 1a and 1 b makes it possiblefor control and monitoring measurements to be made substantially free ofinterference, even during regular telecommunications operation. In thiscontext, control measurements are understood to be those measurementsused for controlling the transmission properties of the transmissionpath. This includes measuring or testing specific fiber parameters,e.g., chromatic dispersion, along with its essential parameters, such aszero dispersion wavelength λ₀, the rise in the dispersion curve at λ₀,or the dispersion values in the second and third optical window, thespectral path attenuation, the polarization mode dispersion, or also biterror rate measurements.

On the other hand, monitoring measurements are measurements which arerelevant to the security of the transmission system. Monitoringmeasurements make it possible, for example, to detect any tampering withthe transmission path. Thus, for example, “eavesdropping” on the datacommunication traffic by tapping off even the smallest amounts of powercan be registered.

In some embodiments of the present invention, neither of the twomeasurement types is subject to any fundamental restrictions,particularly since one can utilize the advantage of the method ofaccommodating all transmitting and measuring devices on the transmitside, so that all data are directly available for analysis. On thereceiver-side, one merely needs to provide for reflection of the controlsignal.

Thus, for example, useful-life measurements can be easily carried out ona continuous basis as interference-free control measurements. Bycomparing the reflection signal and the originally produced controlsignal, one can infer a multiplicity of transmission path parameters,for example, with respect to polarization, propagation time, signalshape, amplitude or intensity. Using a set-up in accordance with FIG. 1,these measurements are able to be performed by selecting a suitabledetecting device, which is able to detect the particular quantities ofinterest.

In addition, as control measurements, it is possible to directly measurethe length of cable fibers. It is known that the length of an opticalfiber is substantially greater than that of a corresponding cable;moreover, the individual fibers of a cable vary in length. Thesemeasurements turn out to be very simple and are carried out, forexample, using a structure of the present invention in accordance withFIG. 3.

FIG. 3 shows a transmission system, which is made up of a transmissionpath 301, over which a transmission signal having wavelength λ₁ istransmitted. The transmission signal comes from a transmitter, in someinstances via another section of the transmission system (other fiberpath). According to the present invention, a control signal havingwavelength λ₂ is transmitted via transmission path 301 to measureproperties of transmission path 301. The control signal can be generatedby an optical time domain reflectometer (OTDR), which is equipped with atransmitter 302 of control wavelength λ₂.

The control signal can be launched, together with the transmissionsignal, by a coupler 306 into transmission path 301. After propagatingthrough transmission path 301, the signal can be reflected off of areflector 305 mounted at the output of transmission path 301, thetransmission signal being transmitted. The alternative illustrated inFIG. 1a may also be used as a reflecting set-up in this embodiment ofthe present invention.

The circulator from the structure according to FIG. 1 is not necessarilyneeded in the optical set-up according to FIG. 3 since the OTDR, asreceiver 303, analyzes the backscattered light having λ₂. A simplebandpass filter 307 upstream from the input or output of the OTDR cansuppress the backscattered radiation having λ₁. The design layoutprinciple is illustrated in FIG. 3.

The OTDR can generate a short control signal having λ₂ and analyze thebackscattered signal. The optical length of the transmission path isdetermined from the time difference between producing the control signaland receiving the reflection signal. Plotting the OTDR measuredbackscattered power logarithmically over the optical path in the testfiber, one obtains a descending straight line (negative slope angle).The fiber end is defined by the occurring Fresnel pulse. Steps in thedescending straight lines indicate spliced joints; the step height is anindex for the splice attenuation. Fiber breakages or defects produceFresnel reflections, which enable them to be precisely pinpointed. Therelation between the measured group propagation time t_(g) and fiberlength L is L=t_(g) c/n_(g), i.e., given exact knowledge of n_(g), L,i.e., each distance between locations in the fiber can be preciselydefined. Using special measuring arrangements, the OTDR can also be usedto measure the locally distributed PMD.

Besides the control measurements, monitoring measurements which arerelevant to security are also able to be simply implemented inaccordance with the present invention without entailing significanttechnical outlay. For monitoring measurements, the design layout inaccordance with FIG. 1 is used, for example.

In a first method, the relative attenuation of the control signal isdetermined on the transmit-side with the aid of detecting device 105.This method may proceed in accordance with the Heitmann measuring method(W. Heitmann, Precision Single-Mode Fiber Spectral AttenuationMeasurements, J. Opt. Comm., vol. 8 (1987), no. 1, p. 2; W. HeitmannAttenuation Analysis of Silica-Based Single-Mode Fibers, J. Opt. Comm.,vol. 11 (1990), no. 4, p. 122).

Because the absolute attenuation values are not the sole focus ofinterest, but rather also and more particularly the small changes inattenuation are, the so-called rear cut-off method is not needed. In therear cut-off method, following a transmission measurement, the fibersare cut off a few meters behind the launching site, and the transmissionpower is measured once again. Highly precise attenuation measurementsare obtained when the launching conditions pertaining to the controlsignal are kept constant (max. 0.001 dB). Every intervention made at thetransmission path in question or at the fibers of the same is detectedby measuring the attenuation. Any tapping of a line for eavesdroppingpurposes causes optical power to be decoupled via the fiber cladding.Even the tapping off of very small optical power of minimally 0.002 dBthat this causes is able to be detected by measuring the attenuation.

In this case, a spectral attenuation measurement is not needed; ameasurement of the control wavelength suffices. The control wavelengthshould be selected to be as large as possible, since large wavelengthsin the monomode range mean that a considerable portion of the opticalpower is conducted in the fiber cladding, so that changes in attenuationcaused by tinkering with the fibers are greater and, therefore, able tobe detected more reliably. However, the control wavelength size selectedshould be such that the attenuation losses over the entire transmissionpath are still justifiable in a normal case, in other words, when thereis no external intervention.

Another method for carrying out a monitoring measurement in accordancewith the present invention is based on the fact that the polarizationstate (SOP) of a signal transmitted via a fiber path is sensitive to thebending and vibration of the fiber: when the fiber is tampered with, itsbirefringence is locally changed, causing the polarization state of thesignal to also change abruptly. This method is by far more sensitivethan the attenuation method and, for the most part, makes it impossiblefor someone to tamper with the cable without it being discovered.

This change in the SOP can easily be verified, for example, using apolarimeter, by plotting the Stokes vector on the Poincaré sphere.Sketched in FIG. 1a,b is the fundamental structure for monitoring pathsby observing the polarization state using a polarization-sensitivemonitoring unit 106. FIG. 4a shows the time characteristic of the threecomponents S₁, S₂, S₃ of the Stokes vector in the undisturbed state; themeasuring time is 8000 s. FIG. 4b is the time characteristic of theStokes components within only 3 s in a disturbance case. Theillustration impressively documents the dramatic effect of disturbanceson the time characteristic of the SOP. This is likewise impressivelyconfirmed by the corresponding plotting on the Poincaré sphere inaccordance with FIGS. 4c and 4 d.

FIG. 4c shows the slow drift of the SOP on the spherical surface in theundisturbed state (measuring time, again 8000 s), while in FIG. 4d, thedisturbance case is shown (measuring time 3 s), in which the Stokesvector fluctuates considerably over the entire spherical surface area.One observes the large difference in the observation time, on the onehand 8000 s, on the other hand only 3 s.

There are several ways to verify the change in the SOP. The mostcomplicated method has been mentioned above. In that method, apolarimeter is used to detect the complete polarization state. In theundisturbed operation, the SOP changes, e.g., as a consequence ofthermal influences, very slowly and steadily (see FIGS. 4a and 4 c). Thepoint on the surface of the Poincaré sphere, which represents theparticular polarization state, slowly drifts in one arbitrary direction.However, if the fiber is touched, the point's position changes suddenly;in response to any tampering with the fiber, the movement of the SOPpoint changes in direction and absolute value virtually randomly (seeFIGS. 4b and 4 d). This process is so conspicuous, that it isimmediately noticeable and detectable.

The sudden change in the SOP due to an external disturbance can bemanifested by an elevated polarization mode dispersion (PMD) and is ableto be detected by measuring the PMD. For this, FIG. 5 shows the spectralpattern of the polarization mode dispersion in ps between 1,540 and1,560 nm (above) without disturbance and (below) with disturbance, whichare caused by the test fiber being contacted; in this case a 23.13 kmlong glass fiber on a coil. In this case, the disturbances at 1,545,1,555 and 1,559 nm can increase the PMD values by a factor of up to 25.FIG. 5 documents the strong influence of disturbances, even on the PMD.As a measuring method in the sense of the work introduced here, thismethod is only considered when the control wavelength is varied using atunable laser, and the time divergence of the signal is plotted as afunction of the wavelength. The requirement to tune the laser makes thistoo time-consuming for routine measurements.

In a second method, ascertaining the change in the SOP does notnecessitate fully defining it by the three scaled Stokes parametersusing a polarimeter. To detect the change in the SOP, it can suffice tomeasure one or two components of the Stokes vector (see FIGS. 4a,b). Forthat reason, to have a less expensive variant, the polarimeter isreplaced by a polarization-sensitive element (polarizer, polarizationbeam splitter, polarization-dependent fiber coupler, etc.). Theradiation concentrations passing through these components are detected,and the corresponding detector voltages analyzed. For as long as thefibers rest undisturbed in the cable, the detector voltage changes onlyvery slowly and steadily. On the other hand, in response to disturbancesor manipulations, there is a sudden and pronounced change. Therefore, todistinguish among these very different effects, it is advantageous touse a discriminator when carrying out automatic monitoring. This can bea frequency filter, for example, which eliminates low frequencies (e.g.,<0.1 Hz). To enhance evaluation security, in addition to the frequencyfilter, a threshold-value switch can be used to prevent small,noise-type voltage values from being analyzed, which also contain veryhigh frequency components. Only amplitudes above the threshold value,which should be able to be variably adjusted, are plotted.

What is claimed:
 1. A method for carrying out control measurements on an optical transmission path, comprising: transmitting a transmission signal via the optical transmission path having a transmit side and a receive side, the transmission signal being transmitted via the optical transmission path having a path form of optical signals having an operational wavelength; transmitting an optical control signal having a control wavelength which is different from the operational wavelength, on the transmit side of the optical transmission path and into the optical transmission path, selecting a coupling mechanism to enable a parallel transmission of the transmission signal; reflecting the optical control signal on the receive side of the optical transmission path; retransmitting the optical control signal over the optical transmission path to the transmit side of the optical transmission path; decoupling the reflection signal formed by the re-reflected optical control signal on the transmit side of the optical transmission path, from the optical transmission path and feeding the reflection signal to a detecting device, where the transmitting of the optical control signal and the transmission signal are substantially unaffected by the decoupling of the reflection signal; emitting an output signal by the detecting device, the output signal being indicative of at least one of intensity state, polarization state, intensity as a function of time, polarization as a function of time and signal propagation time of the reflection signal, and from the output signal deriving transmission properties of at least one of the optical transmission path and changes in the transmission properties; and determining chromatic dispersion of the optical transmission path by analyzing the reflection signal, the chromatic dispersion being at least one of a zero dispersion wavelength, a rise in a dispersion curve at zero dispersion wavelength and dispersion values in one of a second and a third optical window.
 2. The method as recited in claim 1, wherein the decoupling of the reflection signal includes suppressing any decoupled light of the operational wavelength.
 3. The method as recited in claim 1, wherein the method is carried out during normal transmission operation and further comprising launching the control signal in parallel with the transmission signal into the optical transmission path.
 4. The method as recited in claim 1, wherein the reflecting of the control signal on the receive side of the optical transmission path has a reflection coefficient greater than about 95% and the transmitting of the transmission signal has a transmittance greater than about 95%.
 5. The method as recited in claim 1, wherein the control wavelength is selected so that the control wavelength is at least partially transmitted over a cladding of an optical fiber of the optical transmission path, the control wavelength is not substantially attenuated during the transmission.
 6. The method as recited in claim 1, further comprising: determining at least one of a spectral path attenuation and a polarization mode dispersion through analysis of the reflection signal.
 7. The method as recited in claim 1, further comprising: filtering the time-dependent output signal of the detecting device to suppress random fluctuations, the bandwidth of the output signal being about 0.1 Hz to about 100 Hz.
 8. A method for carrying out monitoring measurements on an optical transmission path, comprising: launching a transmission signal via the optical transmission path in the form of an optical control signal having an operational wavelength, the optical transmission path having a transmit side and a receive side; launching the optical control signal on the transmit side into the optical transmission path, the optical control signal having a control wavelength; selecting a coupling mechanism so as to enable a parallel launching of the transmission signal; reflecting the optical control signal on the received side of the optical transmission path and retransmitting the reflected optical control signal to the transmit side over the optical transmission path; decoupling a reflection signal formed by the retransmitted reflected optical control signal on the transmit side from the optical transmission path and feeding the reflection signal to a detecting device, where the decoupling of the reflection signal does at least one of essentially not affecting the launching of the transmission signal and of the optical control signal and suppressing the decoupled light of the operational wavelength; emitting an output signal by the detecting device, the output signal being indicative of at least one of intensity, polarization state, intensity as a function of time, polarization state as a function of time and signal propagation time of the reflection signal, and deriving from the output signal at least one of: transmission properties of the optical transmission path, changes in the transmission properties, and changes in the transmission properties caused by tampering with the optical transmission path; and by analyzing the reflection signal, determining at least one of a chromatic dispersion of the optical transmission path, a zero dispersion wavelength, a rise in a dispersion curve at the zero dispersion wavelength, dispersion values in one of a second and third optical window, a spectral path attenuation, and a polarization mode dispersion. 